In both cases, East should lead the Jack.
If you lead ♠ 5 (normal 4th highest) and declarer plays low, then partner will have to win with ♠ A and South will eventually win the ♠ Q.
If you lead ♠ J, declare will cover with the ♠ Q and partner will win ♠ A. Your ♠ K9 will now “surround” North’s ♠ 10 and South will make no tricks in spades.
Similarly in the club suit.
If you lead ♣ 4 South can let it run to the ♣ 10 and finesse your ♣ K on the next round of the suit.
The opposition make 3 tricks in the suit.
However if you lead ♣ J South will win ♣ Q, but when West gets in to lead another club your ♣ K9 “surrounds” the ♣ 10 and your ♣ 9 will force out the ♣ A.
The opposition now only make 2 tricks in the suit.